The present invention relates generally to the detection of media defects during media production, which is a necessary industrial quality control step in the production process. In the magnetic media industry, media has to undergo glide, burnish and certification before being graded and eventually qualified. The present invention provides an improved testing method which potentially speeds media production.
In media certification, there are tests for Missing Pulse, Extra Pulse, Bit Shift and Modulation. All these tests are run, based on user defined thresholds as a percentage of Track Average Amplitude (TAA). It is therefore necessary to determine Track Average Amplitude before thresholds can be set and tests conducted. Production testing apparatus typically obtain the Track Average Amplitude for a particular track using formulae. This saves time in the derivation of the Track Average Amplitude for every track but the value derived from formulae doe not reflect the true Track Average Amplitude, known as the xe2x80x98true TAAxe2x80x99 of the track. Parametric testing apparatus samples through each track and computes the average of all bytes on that track before using the computed value to compare with every byte on that track. This is xe2x80x98true TAAxe2x80x99, but the method requires a very large amount of memory for storing all of the data for the respective track, a high sampling rate to keep up with disk speed and a lengthy processing time.
A number of different ways are known for determining Track Average Amplitude, depending on the method and type of testing and preferably allow achievement of a minimum xe2x80x98production timexe2x80x99. The following prior art specifications describe different types of testing and the corresponding manner of determining Track Average Amplitude.
U.S. Pat. No. 4,746,995 describes a sequencing operation whereby all tracks are written first and then all tracks are read back to obtain a calculated signal. This has the advantage over writing and reading on a track by track basis in that several drives can be processed simultaneously using the same electronics for one drive. It has the same processing speed as that which uses four carriages on one drive, each doing its write-read in the same sequence. However, the calculated signal is compared with a known standard which means that Track Average Amplitude is not obtained in real time and is not true for every track.
U.S. Pat. No. 5,124,849 discloses writing a series of signal pulses and reading back the pulses using two channels one of which is delayed by a multiple of the pattern""s period. A comparator circuit called the CYCOMP detects amplitude deviation between the two signals; another called the Bandreject circuit detects phase deviation, both of which are indicative of aberrations and are compared with a preset error threshold. The preset error threshold is a percentage of the prevailing value of peak values of the read back signal generated by a peak following means. To collect peak values before establishing the prevailing reference value, one additional revolution for every track is required if a xe2x80x98true TAAxe2x80x99 value is used and this is at the expense of production time.
U.S. Pat. No. 5,280,395 describes a method and an apparatus whereby the mean average value computed based on an average level value for a plurality of tracks prior to the processing track is used as a reference value for comparison with subsequent read back values to detect defects. The process includes a read/write means to write and read back test signals; parametric measurement means for measuring an average-level value for each track based on read back signals: memory means for storing the average-level value for a plurality of tracks prior to the processing track. Arithmetic means for computing an average average-level value based on the average-level values for the plurality of tracks: defect detection means for detecting defects based on the average average-level value compared against read back signal of the processing track. This method gives a more reliable Track Average Amplitude in the form of average average-level value but at the expense of huge processing time. Processing time includes the time to compute average level value for every track prior to the processing track; store the values in memory; compute the average average-value and compare with the read back signal of the processing track.
U.S. Pat. No. 5,532,586 describes an Extra Pulse testing method. An erasing means erases all information on the disk in one revolution. A testing means tests every track for defects. A writing means writes data on the first track before erasure. A reading means read data from the first track before erasure. A data making means calculate from read back signal the reference value to be used for defects detection for the first track. In short there is the initializing process to generate reference data; erasing process to erase all information on disk in one revolution and testing process to test every track for defects. The advantage is that only one revolution is required to erase all information on disk. Again there is no provision for using xe2x80x98true TAAxe2x80x99 for every processing track.
U.S. Pat. No. 4,929,894 describes a method for performing Missing Pulse and Extra Pulse tests simultaneously. This method is intended to enhance throughput for disk drive quality control testing. This method involves a delay means to receive a periodic input signal and to delay that signal for a period equal to one period of the input signal. A difference means determines the difference in amplitudes between the delayed and undelayed input signal. A comparator means compares the difference in amplitudes with a reference signal and generates an error signal should there be difference in amplitudes exceeding the reference threshold. The Missing pulse test is performed at the same time as the undelayed signal is read back and testing time is thus shortened.
To summarise the prior art, the following methods for determining the Track Average Amplitude are described. The method of U.S. Pat. No. 4,746,995 uses a known standard and compares read back signal of every track with the known value. The method of U.S. Pat. No. 5,532,586 performs an initializing process for one track and computes the Track Average Amplitude for that track. That same Track Average Amplitude will be adjusted accordingly for subsequent tracks on the basis of a known relationship defining approximate changes in Track Average Amplitude changes with track number. The method of U.S. Pat. No. 5,280,395 describes the calculation of the average Track Average Amplitude for a plurality of tracks prior to the processing track. The method of U.S. Pat. No. 5,124,849 describes the calculation of a prevailing reference value and there is no account of how that value is computed. The above prior art does not show a method that uses xe2x80x98true TAAxe2x80x99 for every track in the testing nor does it describe a method in which errors are first detected, based on xe2x80x98estimated TAAxe2x80x99, herein called xe2x80x98potential errorsxe2x80x99 following which the xe2x80x98potential errorsxe2x80x99 are verified based on xe2x80x98true TAAxe2x80x99, the verified errors herein called xe2x80x98true errorsxe2x80x99.
According to a first aspect, the present invention provides a method of detecting defects in a selected track of a media element of a mass storage device including the steps of:
a) determining an estimated Track Average Amplitude which approximates the true Track Average Amplitude of a read signal from a track under test of the media element;
b) during first revolution of the media element, writing onto the track under test, a stream of data bits of a predetermined pattern;
c) during a second revolution of the media element, reading back the stream of data bits written to the track under test during the first revolution;
d) comparing the read signal amplitude of each bit of the stream of data read back from the track under test during the second revolution with the estimated Track Average Amplitude;
e) generating an error signal when the amplitude of a bit in the stream of data read back from the track under test during the second revolution falls outside a tolerance band defined relative to the estimated Track Average Amplitude;
f) storing each error signal and a sector at which the error occurs in a storage means;
g) calculating a true Track Average Amplitude from the amplitudes of the stream of data read back from the track under test during the second revolution; and
h) after completion of the second revolution, examining the error signals stored for the track under test, in the storage means and discarding any stored error signals that represent a bit amplitude that did not fall outside of a tolerance band defined relative to the true Track Average Amplitude calculated from the stream of data read back from the track under test during the second revolution.
Preferably, embodiments of the invention include the further step of performing a missing pulse test during second revolution, the comparison threshold for the missing pulse test being determined from the estimated Track Average Amplitude previously obtained.
Preferably also, embodiments of the invention, include the further step of performing a bit shift test during second revolution.
The preferred embodiment also includes the further step of:
a) during third revolution. erasing the data recorded on the track under test during the second revolution; and
b) during fourth revolution, reading back the track under test and comparing the read back signal with the calculated Track Average Amplitude.
In one embodiment of the invention, the step of calculating a true Track Average Amplitude from the amplitude of the bits in the stream of data read back from the track under test during the second revolution is achieved by sampling the data stream, converting each sample to a digital value representing the amplitude of the sample and processing the digital values representing the samples to calculate an average sample value for use as the true Track Average Amplitude.
In another embodiment of the invention, the step of calculating a true Track Average Amplitude from the amplitude of the bits in the stream of data read back from the track under test during the second revolution is achieved by a hardware device to which the read back data is fed the device being arranged to perform an averaging function on the amplitude of the bits of the read back data to produce a signal for use as the true Track Average Amplitude.
Preferably also, the preferred embodiments of the invention relate to production magnetic disks in which a complete revolution of the disk is indicated by index signals which are produced as index marks pass under index mark detectors.
In one embodiment of the invention, the estimated Track Average Amplitude is calculated by an Initialization Process in which data is written onto one track and read back from the same one track to calculate the estimated Track Average Amplitude for all tracks on the disk.
In another embodiment of the invention, the estimated Track Average Amplitude is calculated by a Known Value Process in which an estimated Track Average Amplitude is read from a set of data representing a typical relationship of Track Average Amplitude against tracks.
According to a second aspect, the present invention provides an apparatus for detecting modulation errors on a media element of a mass storage device, the mass storage device having an input/output means which provides access to a head read signal carrying information read from the media element and access to a head write signal allowing input of information to be written onto the media element the apparatus including:
a) envelope detection means to detect the envelope of the head read signal;
b) averaging means to obtain an average of the amplitude of the envelope of the head signals collected over a period of one disk revolution;
(c) an analog to digital converter arranged to periodically sample and digitise local values in the amplitude of the envelope of the head read signal, the analog to digital converter having a conversion status output to signal when conversion of a sample has been completed;
(d) a predictive comparator to compare the amplitude of the local values in the envelope of the head read signal with predetermined thresholds determined from a predicted track average amplitude;
(e) a track error buffer to store a particular digitized sample of the envelope signal when the predictive comparator indicates an out of range value for the particular sample and the conversion status signal indicates that a conversion has been completed;
f) a Sector buffer to store a sector number of each sample saved in the track error buffer.
In a preferred embodiment, the apparatus includes a modulation error detection means arranged to count occurrences of N sequential bit errors and for each occurrence to indicate detection of a modulation error, where N is a user determined. predefined number.
Preferably also, the modulation error detector includes a first counter which increments for every bit error detected and a second counter which increments when the count on the first counter exceeds N to signifying a modulation error, the first counter being reset after every N bits of head signal comparison.
In the preferred embodiment of the invention, Extra Pulse testing is combined with Missing Pulse, Modulation and Bit Shift testings so that all testings for one track are completed in four revolutions (the minimum required for conventional Extra Pulse testing).
The preferred embodiment uses the concept of first detecting errors based on xe2x80x98estimated TAAxe2x80x99 and storing the errors as xe2x80x98potential errorsxe2x80x99 and at the later stage comparing said xe2x80x98potential errorsxe2x80x99 with xe2x80x98true TAAxe2x80x99.
Thus, embodiments of the present invention have the advantage of both being able to use xe2x80x98true TAAxe2x80x99 and to run tests at no extra expense of production time. This is achieved by running Extra Pulse testing concurrently with all the other tests. In addition, within the time required for Extra Pulse testing, which is four disk revolutions, xe2x80x98true TAAxe2x80x99 can be determined during the second revolution and used for Modulation, Missing Pulse and Extra Pulse testing during the third and fourth revolution, respectively.